In recent years, a plasma display panel (hereinafter referred to as a PDP) used for displaying images on a computer or television (TV) has been increasingly required to have not only a larger screen size, smaller thickness, and lighter weight, but also higher definition in order to achieve higher image quality.
A conventional PDP has a typical structure as shown in FIG. 26. With reference to FIG. 26, PDP 1100 is composed of front panel PA1001 and rear panel PA2.
Front panel PA1001 is composed of the following laminated layers: second electrodes, i.e. scan electrodes 19a, first electrodes, i.e. sustain electrodes 19b, and black stripes (a light-blocking layer) disposed in a stripe pattern on front glass substrate 11; dielectric layer 17; and protective layer 1018. Dielectric layer 17 is composed of first dielectric layer 17a and second dielectric layer 17b. First dielectric layer 17a is formed to cover scan electrodes 19a, sustain electrodes 19b, and black stripes 7. Protective layer 1018 is formed on dielectric layer 17. Each scan electrode 19a is made of scan transparent electrode 19a1 and scan metal electrode 19a2. Each sustain electrode 19b is made of sustain transparent electrode 19b1 and sustain metal electrode 19b2.
Rear panel PA2 is composed of the following elements: third electrodes, i.e. address electrodes 14; dielectric layer 13; and barrier ribs 15. The third electrodes, i.e. address electrodes 14, are disposed on rear glass substrate 12 in a stripe pattern. Dielectric layer 13 is formed to cover address electrodes 14. Barrier ribs 15 are formed on dielectric layer 13 in a box shape so as to cover address electrodes 14. Phosphor layers 16 are applied to the inner walls of barrier ribs 15. As the phosphor layers, generally, phosphors in three colors of red, green, and blue are arranged in this order for color display.
Front panel PA 1001 and rear panel PA2 are bonded to each other, and a discharge gas is sealed into discharge part 20 partitioned by barrier ribs 15. For example, a mixed gas composed of helium, neon, argon, krypton, xenon and the like is sealed typically at a pressure of approximately 67 kPa.
Next, a description is provided of an electrode array of the PDP, and a plasma display device that has a driving circuit for driving the PDP. FIG. 27 shows an electrode array of PDP 1100. FIG. 28 is a block diagram showing a structure of circuits for driving the plasma display device. This plasma display device has pane 11001, scan electrode driving circuit 1021, sustain electrode driving circuit 22, address electrode driving circuit 23, timing generating circuit 1024, analog-to-digital (A/D) converter 25, number of scan lines converter 26, subfield converter 27, and averaged picture level (APL) detector 28.
With reference to FIG. 28, image signal VD is input to A/D converter 25. Horizontal synchronizing signal H and vertical synchronizing signal V are input to timing generating circuit 1024, A/D converter 25, and number of scan lines converter 26. A/D converter 25 converts image signal VD into image data of digital signals, and outputs the image data to number of scan lines converter 26 and APL detector 28. APL detector 28 detects the averaged picture level of the image data. According to the averaged picture level detected, driving waveforms forming one TV field are controlled. Number of scan lines converter 26 converts the image data into image data corresponding to the number of pixels of PDP 1100, and outputs the converted data to subfield converter 27. The subfield will be described later. Subfield converter 27 outputs the image data divided into subfields to address electrode driving circuit 23. Address electrode driving circuit 23 applies voltages corresponding to address electrode D1 through address electrode Dm to the address electrodes for each of the subfield.
Timing generating circuit 1024 generates timing signals based on horizontal synchronizing signal H and vertical synchronizing signal V, and outputs the timing signals to scan electrode driving circuit 1021 and sustain electrode driving circuit 22. Scan electrode driving circuit 1021 and sustain electrode driving circuit 22 apply driving voltages to scan electrode SCN1 through scan electrode SCNn, and sustain electrode SUS1 through sustain electrode SUSn, respectively, according to the timing signals.
Next, a description is provided of gradation representation method used in PDP 1100. FIG. 29 shows a gradation representation method used in PDP 1100. When a TV image is displayed, an image compliant with National Television System Committee (NTSC) system, for example, is formed of 60 fields per second. Originally, PDP 1100 is capable of representing only two levels of gradation, i.e. light emission and non-light emission. Thus neutral colors are represented in the following manner. One field period is divided into a plurality of subfields (hereinafter, SFs). The periods for emitting red, green, and blue light are time-divided and combined. For example, the numbers of sustain pulses applied in the discharge sustain periods of the respective SFs are weighted to have ratios in a binary mode, such as 1, 2, 4, 8, 16, 32, 64, and 128. SFs of the 8-bit combination can provide 256 levels of gradation representation.
In this method, each SF is further divided into four periods so that the gas discharge in discharge part 20 is controlled. FIG. 30 shows voltage waveforms applied to scan electrodes SCN, sustain electrodes SUS, and address electrodes D in one SF for driving the plasma display device. These four periods will be described with reference to FIG. 26, FIG. 27, and FIG. 30.
In an initializing period, prior to address period 1032 in which an address discharge for selecting cells to be lit is performed, wall charges desired for the address discharge are accumulated by a weak discharge. In the first SF in one TV field, all-cell initializing period 1031 is set. In this all-cell initializing period, an all-cell initializing operation for causing an initializing discharge in all the cells used for displaying an image is performed. In the other SFs, selective initializing periods 1034 are set. In this selective initializing period, the all-cell initializing operation or a selective initializing operation is performed. In the selective initializing operation, the initializing discharge is caused only in the cells having undergone a sustain discharge in the preceding SF. In address period 1032, cells to be lit by an address discharge are selected. In sustain period 1033, a sustain operation for sustaining light emission in the cells having undergone the address discharge in address period 1032 is performed.
In the initializing operation in the first half of all-cell initializing period 1031, all the sustain electrodes, i.e. sustain electrode SUS1 through sustain electrode SUSn, and all the address electrodes, i.e. address electrode D1 through address electrode Dm, are kept at 0 V. To all the scan electrodes, i.e. scan electrode SCN1 through SCNn, a ramp voltage gradually rising toward voltage Vh is applied. Here, voltage Vh is equal to or higher than threshold voltage Vff at which a discharge starts between the scan electrodes and sustain electrode SUS1 through sustain electrode SUSn in pairs with the scan electrodes, and between the scan electrodes and address electrode D1 through address electrode Dm faced to the scan electrodes. Thus a gas discharge occurs in discharge part 20. This discharge is a weak discharge in which electrolytic dissociation temporally gradually proceeds. The electric charges generated by this weak discharge are accumulated on the wall surfaces surrounding discharge part 20 so as to reduce the electric field of the inside and surfaces of discharge part 20 in the periphery of address electrodes 14, scan electrodes 19a, and sustain electrodes 19b. A negative charge is accumulated on the surface of protective layer 18 in the vicinity of scan electrodes 19a. A positive charge is accumulated on the surface of protective layer 18 in the vicinity of sustain electrodes 19b and the surface of phosphor layers 16 in the vicinity of address electrodes 14.
Further, in the initializing operation of the second half of all-cell initializing period 1031, all the sustain electrodes, i.e. sustain electrode SUS1 through sustain electrode SUSn, are kept at positive voltage Ve. To all the scan electrodes, i.e. scan electrode SCN1 through SCNn, a ramp voltage gradually falling toward voltage Vbt is applied. Here, voltage Vbt is equal to or lower than threshold voltage Vpf at which a discharge starts between the scan electrodes and sustain electrode SUS1 through sustain electrode SUSn in pairs with the scan electrodes, and between the scan electrodes and address electrode D1 through address electrode Dm faced to the scan electrodes. Thus a gas discharge occurs in discharge part 20. This discharge is also a weak discharge in which electrolytic dissociation temporally gradually proceeds. This weak discharge reduces the negative charge accumulated on the surface of protective layer 18 in the vicinity of scan electrodes 19a and the positive wall charge accumulated on the surface of protective layer 18 in the vicinity of sustain electrodes 19b. 
In a state where all the electrodes are grounded after completion of the all-cell initializing operation, a potential difference (hereinafter referred to as a wall potential) necessary for selecting cells to be lit by the address discharge is generated by the accumulated wall charges, between the scan electrodes and address electrodes 14 and between the scan electrodes and sustain electrodes 19b. The initializing operation is an operation in which a discharge forms wall charges desired for controlling the address discharge.
In address period 1032, scan electrodes 19a are applied with a voltage lower than those applied to address electrodes 14 and sustain electrodes 19b. Further, only address electrodes 14 in the cells to be lit are applied with a voltage so that a voltage difference of the same polarity as the wall potential is generated between scan electrodes 19a and address electrodes 14. This voltage application causes an address discharge. Thus, as a wall charge, a negative charge is accumulated on the surface of the phosphors and the surface of the protective layer in the vicinity of sustain electrodes 19b. A positive charge is accumulated on the surface of the protective layer in the vicinity of scan electrodes 19a. In a state where the address period is completed and all the electrodes are grounded, a desired wall potential at which the wall charges cause a sustain discharge between scan electrodes 19a and sustain electrodes 19b is generated.
In sustain period 1033, first, scan electrodes 19a are applied with a voltage higher than the voltage applied to sustain electrodes 19b, and thus a discharge occurs between the electrodes. Thereafter, the voltage is applied to scan electrodes 19a and sustain electrodes 19b so that the polarity is alternately changed. Thus the light emission is intermittently sustained.
In subsequent selective initializing period 1034, at the end of sustain period 1033 of the preceding SF, sustain electrodes 19b are applied with an erasing voltage in a rectangular waveform so that a short time difference is provided from the voltage application to scan electrodes 19a. Such voltage application causes an incomplete discharge and erases a part of the wall charges, which are preparations for the initializing operation in the subsequent SF. In this manner, in the conventional method for driving a PDP, images are displayed in a sequence of the initial period, address period, and sustain period. The all-cell initializing operation is performed not only in the first SF of one field, and can be performed in other SFs.
For PDP 1100 of FIG. 26, in all-cell initializing period 1031 in which desired wall charges are accumulated by a weak discharge, the density of ions or electrons (charged particles, i.e. a source of electrolytic dissociation) present in discharge part 20 at the initial stage thereof is low, or the phosphors or barrier ribs likely to absorb the electric charges of charged particles surround discharge part 20. In such cases, the number of the charged particles, i.e. a source of discharge, is absolutely decreased. This state increases the probability of occurrence of strong discharge in which electrolytic dissociation temporally rapidly proceeds (hereinafter, referred to as a strong discharge).
When a strong discharge occurs, wall charges (e.g. wall charges substantially canceling out the electric field of discharge part 20) more than the desired wall charges are accumulated, and thus an abnormal wall potential higher than the desired wall potential is generated.
The action of this abnormal wall potential causes sustain light emission in the cells to be unlit in the sustain periods, and images cannot be displayed normally (see Patent Document 1, for example).
Further, image display using a high-definition PDP has the following problems. For example, in a high-definition PDP, even when the cells are isolated by barrier ribs, a fine cell pitch (intervals between barrier ribs) increases the influences of the electric field interference between the adjacent cells and scattering charged particles.
In the conventional method for driving a PDP as shown in FIG. 30, application of a rectangular waveform voltage in selective initializing period 1034 causes a strong erasing discharge. Thus, when a high-definition PDP is driven, the influence of discharge interference between the adjacent cells in the initializing operation is conspicuous. Therefore, wall potential desired for the address operation cannot be accumulated, and the address operation cannot be performed normally (see Patent Document 2, for example).
In the conventional PDP, the amount of electrons to be supplied for causing a stable initializing operation is insufficient in the following two cases, for example. The pixel pitch is reduced for higher definition, and thus in discharge part 20, the rate of the surface area to the volume is increased. For a higher luminance, the mixing ratio of a gas having a larger atomic number, e.g. xenon and krypton, in the discharge gas is increased. In such cases, a strong discharge occurs in the initializing periods. The abnormal wall charges accumulated by the strong discharge cause sustain light emission in the cells to be unlit in the sustain periods. As a result, images cannot be displayed normally.
Further, when a high-definition PDP is driven by the conventional driving method, the influences of the electric field interference between the adjacent cells and scattering charged particles are conspicuous in a selective initializing period. Thus, no sustain light emission is caused in the cells to be lit in the sustain periods, and images cannot be displayed normally.
The reasons why these problems become more conspicuous as the definition is increased will be detailed hereinafter.
As the definition is increased, the volume of each cell in discharge part 20 is decreased. Thus the rate of the surface area of the wall surface to the volume of discharge part 20 is increased. This structure increases the energy loss caused by heat generation resulting from the re-absorption and elastic collision of the charged particles on the wall surfaces. The energy loss necessitates introduction of more electric power externally. As a result, the number of charged particles inside discharge part 20 before the all-cell initializing operation is decreased, and the driving voltage in each period is increased.
When an increased voltage is applied to electrodes, the field intensity increases in the inside and surfaces of discharge part 20 in the periphery of the electrodes. Thus the probability that electrolytic dissociation temporally rapidly proceeds is increased. As a result, it becomes more difficult to generate a weak discharge used for the conventional initializing operation.
In this manner, with an increase in definition, the charged particles inside of discharge part 20 are decreased and the driving voltages are increased. Thus a strong discharge is more likely to occur in the initializing periods. As a result, it becomes more difficult to normally select the cells to be lit or the cells to be unlit in the address periods.
Further, with an increase in definition, the size of each cell is reduced. This size reduction increases the light-blocking rate determined by the barrier ribs and metal electrodes, and decreases the luminance. Thus the images become darker in general. To address this problem, as a method for ensuring a luminance necessary for displaying a high-quality image, a method for increasing the mixing ratio of xenon or krypton causing emission of visible light, or the total pressure of a discharge gas is drawing attention. For example, total pressures from 180 Torr to 750 Torr inclusive, and xenon partial pressures of 10%, 15%, 20%, 30%, 50%, 80%, 90%, 95%, 98%, and 100% are considered.
The reason why the above problems are conspicuous at a larger mixing ratio of xenon, krypton, or the like will be detailed hereinafter.
In an element having a larger atomic number, such as xenon and krypton, the electron energy (first ionization energy) in the outermost shell is small. Thus the secondary electron emission coefficient of such an element is extremely smaller than those of helium, neon, and argon that have larger electron energies in the outermost shells. As a result, the absolute number of electrons supplied from the surface of the protective layer to discharge part 20 is small, and the threshold voltage necessary for starting discharge is high.
When an increased voltage is applied to electrodes, the field intensity increases in the inside and surfaces of discharge part 20 in the periphery of the electrodes. This phenomenon increases the probability that electrolytic dissociation temporally rapidly proceeds. As a result, it becomes more difficult to generate the weak discharge used in the initializing period.
When the partial pressure of xenon or krypton is increased to ensure a high luminance necessary for displaying high-quality images, a strong discharge is likely to occur in the all-cell initializing operation. When a strong discharge occurs, one discharge provides high emission intensity. Thus the contrast ratio is considerably decreased, and the image quality is considerably degraded when images having many low gradation representations are displayed. Further, formation of excessive wall potential makes it more difficult to select the cells to be lit or the cells to be unlit in the address periods.    [Patent Document 1] Japanese Patent Unexamined Publication No. 2000-214823    [Patent Document 2] Japanese Patent Unexamined Publication No. 2006-151295